The present invention is directed to a spark plug having a center electrode and a ground electrode. A portion of at least one of the center electrode and ground electrode includes a spark portion having a base material and a protective material to prevent corrosion of the base material.
Spark plugs are well known in the industry and have long been used to initiate the combustion in internal combustion engines. Spark plugs perform the basic function of igniting gases in an engine cylinder, the ignition of which creates the power stroke. Due to the very nature of internal combustion engines, spark plugs are exposed to many extremes occurring within an engine cylinder including high temperatures and various corrosive combustion gases which traditionally have reduced the longevity of the spark plug. Spark erosion may also reduce the longevity of the spark plug.
Electrical spark erosion is where the electrode and, in particular, the firing tip of a spark plug, erodes away during operation due to the periodic energy of the spark arc vaporizing the electrode material. Spark plugs traditionally have electrodes formed from Nickel or Nickel alloys which are susceptible to spark erosion. The use of new technology in engines to improve fuel economy has resulted in increased energy passing through the spark plug to force the spark to jump the gap between the center electrode and ground electrode and potentially a longer arc duration. This increased energy has increased the rate of spark erosion in materials susceptible to spark erosion and more spark plug manufacturers are turning away from commonly used Nickel or Nickel alloy materials in search of materials that are highly resistant to spark erosion such as Platinum, Iridium, or alloys thereof.
While Nickel and Nickel alloys traditionally have been very resistant to corrosion, many of the replacement metals or metal alloys, which are more resistive to spark erosion than Nickel or Nickel alloys, may also be susceptible to corrosion. The most common replacement materials for Nickel or Nickel alloys have been Platinum, Iridium, or alloys thereof. As Platinum and Iridium are generally expensive, it is desirable to minimize the amount of material used to provide the spark portion. Therefore, a spark portion formed out of Platinum or Iridium or alloys thereof is typically attached to a Nickel or Nickel alloy center electrode and minimized in size.
While Platinum and Platinum alloys are very good at reducing spark erosion, they may also be susceptible to corrosion. Furthermore, Platinum and Platinum alloys when used as the spark portion may alloy with combustion constituents and may form nodules or growths on the spark portion. Over time these growths may eventually interfere with the spark or change the spark gap or spark profile thereby reducing the performance of the spark plug. Furthermore, as some of the combustion gases may cause corrosion of the Platinum spark portion, such corrosion may cause the spark plug gap to change and thereby reduce the performance of the spark plug. Reduced performance of spark plugs can cause engine misfire, decreased fuel economy, and poor engine performance.
To improve performance of spark plugs and prevent growth of various materials on the spark portion of the spark plug, many manufacturers of spark plugs have recently been switching to Iridium as the discharge or spark portion. As Iridium has a very high melting point, it is also highly resistant to spark erosion but it is susceptible to oxidation and other corrosion at higher operating temperatures. However, as engine manufacturers increase electrical and thermal stresses to the spark plug through engine changes to improve fuel economy, it has been found that Iridium has a very volatile oxidation state at high temperatures, such as the upper end of the operating range of the spark plug (800-1100° C.) In comparison to traditional engines, these newer technology engines require more energy to be supplied through the spark plug to force the spark to jump the gap between the center electrode and ground electrode, and the operational temperature of the spark plugs has been increasing. At high temperatures an Iridium spark portion of a spark plug may experience severe corrosion.
In one particular mode, corrosion of the Iridium is believed to occur when Calcium and/or Phosphorus react with the Iridium to cause corrosion and erosion of the spark portion. The presence of Calcium and Phosphorus in combustion materials is a relatively more recent development as engine manufacturers attempt to increase fuel economy by reducing friction, and therefore, sometimes allowing more oil to seep into the combustion chamber. Calcium and Phosphorus are primarily present in engine oils and, in particular, oil additives. It is believed that Calcium and Phosphorus in the presence of oxygen during combustion within the engine cylinder react with the Iridium to form a volatile compound that evaporates and results in the loss of Iridium in the spark portion. More specifically, it is believed that gaseous Calcium during the combustion and exhaust cycle condenses on the Iridium spark portion of the spark plug and, in particular, the sides of the spark portion. It is known the molten Calcium dissolves Iridium and that Iridium is vulnerable to oxidation in the presence of Phosphorus. Therefore, the compound formed after the Phosphorus and oxygen react with the dissolved Calcium Iridium mixture is very volatile and subject to evaporation or vaporization which results in loss of the Iridium spark portion. More specifically this mechanism of corrosion with Phosphorus and Calcium typically corrodes the sides of the electrode, and not the spark surface facing the opposing electrode, which due to the activity of the spark on the spark surface is believed to prevent the accumulation of corrosive deposits. A diagram of a spark plug showing the loss of a portion of the spark portion is shown in FIG. 1. It should also be noted that Iridium may also experience some oxidation without the presence of Calcium and Phosphorus in the temperature range of about 800 to 1100° C. and with the presence of Calcium and Phosphorus the above described corrosion process may occur as low as 600° C., which is within the typical operating range of a spark plug. Of course, as engine compression increases, the temperature operating range of a spark plug will increase and oxidation of Iridium even without the presence of Calcium and Phosphorus will increasingly become a problem.